The commercial polymerization of ethylene using coordination catalysts can be carried out in the high pressure, solution, slurry (suspension) or gas phase polymerization processes. The slurry and gas phase processes are examples of the so called particle form processes. In such systems, the catalyst for the polymerization is typically supported on an inert carrier. The polymerization is then carried out at temperatures below the melting point of the polymer, thereby precipitating the polymer onto the carrier. This results in the polymer powder particles growing while being suspended in either a diluent (slurry) or a fluidized polymer bed (gas-phase). The relatively low polymerization temperatures of these processes allows the manufacturer to produce polymers of very high molecular weight.
The most common ethylene polymerization catalysts are the chromium-based (so-called Phillips type) catalysts supported on silica(Cr—SiO2), or the titanium based (so-called Ziegler type) catalysts supported on magnesium chloride (MgCl2) and/or silica. However the relatively recent introduction of metallocene-based single site catalysts for ethylene/α-olefin copolymerization has resulted in the production of new ethylene interpolymers (the term “interpolymer” is used herein to indicate a polymer wherein at least two different monomers are polymerized to make the interpolymer including copolymers, terpolymers, etc.). These single site catalysts include the bis(cyclopentadienyl)-catalyst systems as described by Hlatky et al in U.S. Pat. No. 5,153,157 and the constrained geometry catalysts. These catalysts and methods for their preparation are disclosed in U.S. application Ser. No. 545,403, filed Jul. 3, 1990 now abandoned (EP-A-416,815), European Patent Application EP-A-468,651; European Patent Application EP-A-514,828; U.S. application Ser. No. 876,268, filed May 1, 1992, now U.S. Pat. No. 5,721,185 (EP-A-520,732) as well as, U.S. Pat. No. 5,374,696, U.S. Pat. No. 5,470,993; U.S. Pat. No. 5,055,438, U.S. Pat. No. 5,057,475, U.S. Pat. No. 5,096,867, U.S. Pat. No. 5,064,802, and U.S. Pat. No. 5,132,380. In addition, certain cationic derivatives of the foregoing constrained geometry catalysts that are highly useful as olefin polymerization catalysts are disclosed and claimed in U.S. Pat. No. 5,132,380. In U.S. Pat. No. 5,453,410 combinations of cationic constrained geometry catalysts with an alumoxane were disclosed as suitable olefin polymerization catalysts. For the teachings contained therein, the afore-mentioned pending U.S. patent applications, issued United States Patents and published European Patent Applications are herein incorporated in their entirety by reference thereto.
In order for such catalysts to be used in the particle form processes, supported versions of constrained geometry catalysts have also been developed, such as those disclosed in WO 96/16092 and WO 96/28480 (the teachings contained therein, are herein incorporated in their entirety by reference). In these systems, the active form of the catalyst is strongly associated with the support and thus has no possibility of diffusing into the diluent during typical slurry process polymerization conditions.
A feature of these catalyst composition is the preparation of a solid component which can, as in WO 96/16092, comprise;
1) a silica support and an alumoxane in which the alumoxane is fixed to the support material by a heating and/or washing treatment, such that the alumoxane is substantially not extractable under severe conditions (toluene at 90° C.); and
2) a constrained geometry complex.
When the amount of extractable alumoxane is low, little can diffuse into the polymerization solvent or diluent if used, and thus little or no activation of the catalyst occurs in the diluent. Thus no appreciable amount of polymer will be formed in the diluent, as compared to polymer formed on the support material. If too much polymer is formed in the diluent the polymer bulk density will decrease below acceptable levels and reactor fouling problems may occur.
Alternatively, as in WO 96/28480, the solid (or supported) catalyst can be formed from;
1) a silica support material, which is treated with an organometallic metal alkyl compound (selected from Groups 2-13 of the Periodic Table of the Elements, germanium, tin, and lead); and
2) an activator compound which comprises a cation (which is capable of reacting with a transition metal compound to form a catalytically active transition metal complex) and a compatible anion having up to 100 nonhydrogen atoms and containing at least one substituent comprising an active hydrogen moiety; and
3) a constrained geometry complex.
The activator compound reacts with the metal alkyl compound through the active hydrogen-containing substituent. It is believed that the alkyl group of the metal alkyl compound combines with the active hydrogen moiety of the activator compound to release a neutral organic compound, for example an alkane, or hydrogen gas thereby chemically coupling the metal atom with the activator compound residue. Thus the activator is believed to become chemically attached to the support material. Upon addition of the constrained geometry complex, a supported catalyst is formed in which the active form of the catalyst is strongly associated with the support and remains there during the course of the polymerization conditions.
Additional advantages of polymers produced by single site catalysts as opposed to the more traditional Ziegler or Phillips (chromium on silica) catalysts relate to the nature of the catalyst residues remaining in the polymer after polymerization. Ziegler catalysts are typically prepared from chloride complexes of titanium such as TiCl4 or TiCl3 and are often supported on magnesium chloride (MgCl2). Thus polymers produced by Ziegler catalysts often contain significant concentrations of magnesium chloride and other chloride containing catalyst residues in the polymer products. This can adversely effect the appearance of the film (due to “fish eye” formation) or cause corrosion problems with processing equipment. Similarly the products of the Phillips type (Cr on SiO2) systems can contain chromium residues which if found at too high a level can limit the use of such resins for example in food contact applications
Removal of such catalyst residues from the polymer can require the addition of expensive and time consuming post reactor polymer processing steps such as steam stripping or other methods of washing out the catalyst residues prior to polymer fabrication or sale.
The single site, and especially, the constrained geometry catalysts however are substantially chloride and chromium and free and, when supported, are usually supported on silica and not magnesium chloride and thus the resulting polymers have much lower (often zero) levels of chloride- and chromium-containing catalyst residues in their products, even in the absence of additional post reactor polymer washing steps
Conventional supported Ziegler catalysts tend to produce polymers with relatively broad molecular weight distribution which varies with Mw. For instance Bailey et al. (U.S. Pat. No. 4,547,551, Phillips Petroleum) report that for a polymer with having an Mn around 70000, produced by a magnesium chloride-supported Ziegler system with an organoaluminum cocatalyst, the Mw/Mn is around 7.5. For ethylene homopolymers produced by Ziegler catalysts, the polymer density is also dependent upon molecular weight. The entropy that has to be overcome by crystallizing a longer homopolymer molecule is higher and more difficult to overcome than for a shorter homopolymer molecule. Consequently homopolymer density tends to decrease with increasing molecular weight. A typical Ziegler-Natta homopolymer having a melt index (I2) of approximately 1 g/10 min will have a density higher than 0.960 g/cm3.
The molecular weight distributions of homopolymers prepared using most supported or unsupported single site catalysts, which are not of the constrained geometry type, are smaller or equal to 3 over the complete molecular weight range. In addition, the density of such homopolymers is typically lower than the densities of the analogous homopolymers of the same molecular weight, prepared using Ziegler catalysts. This is exemplified by Stehling et al. (U.S. Pat. No. 5,382,631) which discloses, in sample designation '006 of Example 2, that a homopolymer, prepared under gas phase conditions using a supported single site catalyst, and having a melt index (I2) of 5.0 g/10 min, has a density of only 0.9552 g/cm3 and an Mw/Mn of 2.80. Similarly Lux et al in Example 12 of WO 95/18160 using a supported single site catalyst disclose that a homopolymer, prepared under slurry process conditions, and having a melt index (I2) of 0.2 g/10 min, has a density of only 0.9450 g/cm3 and an Mw/Mn of 2.77. This can be contrasted with the a typical Ziegler catalyst homopolymer product having an I2, of 1.0 g/10 min, which will have a density greater than 0.9600 g/cm3 and an Mw/Mn much greater than about 3.
In many applications, it is highly desirable for a homopolymer to have a high density for improved toughness and stiffness. It is also highly desirable for such a high density homopolymer to have a relatively low Mw/Mn (i.e. less than about 5) at low molecular weights (i.e. less than about 100,000). This minimizes the wax content of the polymer which otherwise can lead to die wax build up and smoke generation on extrusion and taste and odor problems in the resulting fabricated articles. It is also highly desirable for such a high density homopolymer to have a broader Mw/Mn (i.e. greater than about 4) at higher molecular weights (i.e. greater than about 100,000) as an aid to processability of the polymer.
Thus homopolymers produced from Ziegler catalysts have the disadvantage of typically exhibiting a broad Mw/Mn especially at low molecular weights.
Homopolymers derived from typical single site catalysts have the dual disadvantage of;
a) being unable to attain as high a density for a given molecular weight as comparable Ziegler products and;
b) exhibiting a narrow Mw/Mn across the whole molecular weight range (which can limit processability especially at high polymer molecular weights).
Thus there remains a requirement for the production of ethylene homopolymers which, while having a high density, also have a narrow Mw/Mn at low molecular weight and a broader Mw/Mn at higher molecular weight.
Other uses of ethylene homopolymers involve their use as one of the components of blend compositions. It is known that improvement in impact and, environmental stress crack resistance (ESCR) of an ethylene copolymer, can be achieved by decreasing the comonomer content of the low molecular weight fraction of the ethylene copolymer to a level as low as possible while increasing the comonomer content of the high molecular weight fraction of the ethylene copolymer to a level as high as possible. It has also been demonstrated (as for example by Zhou et al, Polymer, Vol 24, p. 2520 (1993)), that large strain properties such as toughness tear, impact and ESCR can also be improved by the presence of “tie molecules” in the resin. High molecular weight molecules with the highest comonomer content (i.e. the highest degree of short chain branching) are responsible for the formation of most of the tie molecules upon crystallization.
Thus attempts to maximize properties such as toughness, modulus, impact strength and ESCR, without sacrificing processability, has resulted in the preparation and use of blend compositions made out of two or more polymer components of differing molecular structures. Blends containing solely Ziegler catalyst products are described in a number of patents.
For example, Nelson (U.S. Pat. No. 3,280,220, Phillips Petroleum) teaches that a blend of an ethylene homopolymer of low molecular weight (formed in the solution process) and an ethylene-butene-1 copolymer of high molecular weight (formed in a particle form process) provides higher ESCR advantageous for containers (bottles) and pipe than similar blends of copolymers.
Hoblitt et al. (U.S. Pat. No. 3,660,530, The Dow Chemical Company) teaches a method where part of the homopolymer produced after a first reaction step is subjected to 1-butene. The still active catalyst then produces a block copolymer of polyethylene and polymerized butene-1. Both components are then admixed. The resultant blend has improved ESCR properties.
Fukushima et al. (U.S. Pat. No. 4,438,238) disclose blends consisting of components with densities between 0.910 and 0.940 g/cm3 and broad molecular weight and blend distributions substantially not having long chain branches have been found to have good processability similar to high pressure polyethylene
Bailey et al. (U.S. Pat. No. 4,547,551) teach that ethylene polymer blends of a high molecular weight ethylene polymer, preferably an ethylene-mono-α-olefin copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, both preferentially with a narrow molecular weight distribution and low levels of long chain branching exhibit excellent film properties and a better balance of stiffness and impact and environmental stress cracking resistance (ESCR), superior to that expected for polyethylene of comparable density and flow.
Morimoto et al. (U.S. Pat. Nos. 5,189,106, and 5,260,384) disclose blends consisting of a high molecular weight copolymer in combination with a low molecular weight homopolymer have been found to possess good processability and excellent low temperature mechanical properties.
Boehm et al., (Advanced Materials 4 (1992) No 3, p 237), disclose the cascade polymerization process in which the comonomer is introduced in the high molecular weight fraction of the polymer resulting in a larger amount of comonomer being present at the same overall density. This in turn results in a polymer composition having improved rigidity-lifetime (failure time) compared to conventional unimodal copolymers. Several patents have also appeared teaching the process to produce such materials in a cascade process such as EP 0 022 376 (Morita et al).
Finally, Sakurai et al (U.S. Pat. No. 4,230,831) disclose that it is beneficial to mix low density polyethylene with various blend compositions to improve polymer die swell or melt tension.
Blend compositions of homogeneous interpolymers having narrow molecular weight distribution and narrow composition distributions are also known. Stehling et al. in U.S. Pat. Nos. 5,382,630 and 5,382,631 describe polymer compositions made by blending components which have Mw/Mn of less than 3 and a Composition Distribution Breadth Index of ≧50%. The components are said to be produced by using metallocene catalyst systems known to provide narrow composition distributions and narrow molecular weight distributions.
Blend compositions containing both Ziegler and single site catalyst products have also been disclosed. Research Disclosure No. 310163 (Anonymous) teaches that blends of Ziegler Natta- and metallocene-catalyzed ethylene copolymers when fabricated into cast films have improved optical, toughness, heat sealability, film blocking and unwind noise properties when compared with metallocene-catalyzed polymer alone.
Research Disclosure No. 37644 (Anonymous) teaches that blends of traditionally (Ziegler-Natta) catalyzed resins and resins made by single site metallocene catalysts display superior transverse direction tear and machine direction ultimate tensile properties useful in cast film applications.
WO 94/25523 (Chum et al.) teaches that films having synergistically enhanced physical properties can be made, when the film is a blend of at least one homogeneously branched ethylene/α-olefin interpolymer and a heterogeneously branched ethylene/α-olefin interpolymer. Films made from such formulated compositions have surprisingly good impact and tensile properties, and an especially good combination of modulus and toughness.
However, blends derived totally from Ziegler catalyzed products still have the problem that the low molecular weight component will generate a high amount of extractables because of the broad MWD, and the high molecular weight component does not have the desirable comonomer distribution to generate a high tie molecule distribution, although the molecular weight distribution is broad. Blends derived from products prepared using conventional supported single site catalysts are limited in the overall density that they can achieve for a given total comonomer content at a final molecular weight, relative to blends containing Ziegler catalyzed materials, as the traditional single site catalysts are unable to generate as high a homopolymer density for a given molecular weight as the Ziegler catalyzed materials.
However, for blends containing both single site and Ziegler catalyst products, if the low molecular weight homopolymer blend component is produced using a Ziegler catalyst, the homopolymer density will be high but its molecular weight distribution (Mw/Mn) will be broad leading to a high amount of extractables. If a narrow molecular weight distribution (Mw/Mn) single site catalyst product is used as the high molecular weight component of the blend it will not be capable of generating the same amount of tie molecules, because of the lack of very high molecular weight molecules, also its comonomer distribution will not be optimal. Conversely, if the low molecular weight component is a homopolymer produced with single site catalyst, the homopolymer density cannot be increased as desired. Also, the comonomer distribution of the high molecular weight Ziegler Natta material is not optimal, although its molecular weight distribution is broad.
There also remains a requirement for blend compositions which have a low molecular weight homopolymer component having a high density and an Mw/Mn which increases with molecular weight, and a higher molecular weight component having an overall high comonomer content and wherein the lower the molecular weight of a copolymer fraction in the molecular weight distribution of a said higher molecular weight component, the lower the comonomer content of the copolymer fraction; and, in the other aspect, the higher the molecular weight of a fraction of said higher molecular weight component, the higher the comonomer content of the copolymer fraction.
Finally there also remains a requirement for the production of ethylene homopolymers and blend compositions which exhibit excellent stiffness and toughness with good ESCR, impact and modulus and exhibiting excellent processability while minimizing wax buildup on the die, smoke generation on the extruder on processing, and low extractables in the resin to minimize its taste and odor